Process for the preparation of acetic anhydride

ABSTRACT

Acetic anhydride is prepared from methyl acetate and/or dimethyl ether in carbonylation processes comprising the use of a halide, carbon monoxide and a Group VIII noble metal in the presence of promoters comprising metallic hafnium and an organo-phosphorus compound or an organo-nitrogen compound wherein the phosphorus and nitrogen are trivalent, or an arsine of the formula: ##STR1## wherein R and R 1  are monocyclic aryl groups or alkyl groups and R 2  is the radical ##STR2## a monocyclic aryl group or an alkyl group, and wherein R 3  and R 4  are each a monocyclic aryl group or an alkyl group, n being zero or a digit from 1-20.

This invention relates to the preparation of acetic anhydride bycarbonylation.

Acetic anhydride has been known as an industrial chemical for many yearsand large amounts are used in the manufacture of cellulose acetate. Ithas commonly been produced on an industrial scale by the reaction ofketene and acetic acid. It is also known that acetic anhydride can beproduced by the decomposition of ethylidene diacetate, as well as by theoxidation of acetaldehyde, for example. Each of these "classic"processes has well-known drawbacks and disadvantages and the search foran improved process for the production of acetic anhydride has been acontinuing one. Proposals for producing anhydrides by the action ofcarbon monoxide upon various reactants (carbonylation) have beendescribed, for example, in Reppe et al. U.S. Pat. Nos. 2,729,561,2,730,546 and 2,789,137. However, such prior proposals involvingcarbonylation reactions have required the use of very high pressure.Carbonylation at lower pressures has been proposed but as a route to thepreparation of acetic acid. French Pat. No. 1,573,130, for example,describes the carbonylation of methanol and mixtures of methanol withmethyl acetate in the presence of compounds of iridium, platinum,palladium, osmium and ruthenium and in the presence of bromine or iodineunder more moderate pessures than those contemplated by Reppe et al.Similarly, South African Pat. No. 68/2174 produces acetic acid from thesame reactants using a rhodium component with bromine or iodinecomponents. Schultz (U.S. Pat Nos. 3,689,533 and 3,717,670) hasdisclosed a vapor-phase process for acetic acid production employingvarious catalysts comprising a rhodium component dispersed on a carrier.

More recently, Belgian Pat. No. 819,455 shows the carbonylation ofcertain esters and/or ethers to produce carboxylic acid anhydridesemploying Group VIII noble metal catalysts in the presence of bromine oriodine moieties optionally in the presence of promoters comprising atleast one metal which is an element having an atomic weight greater than5 of Groups IA, IIA, IIIA, IVB, and VIB, a non-noble metal of Group VIIIor a metal of the lanthanide and actinide groups of the Periodic Table,and their compounds. U.S. Pat. No. 3,927,078 shows the preparation ofacetic anhydride by the carbonylation of methyl acetate or dimethylether in the presence of a Group VIII noble metal compound and in thepresence of an iodide or bromide promoter and is characterized by theinclusion of a proton donor in the reaction system. The optional use ofLewis acids is also disclosed. U.S. Pat. No. 4,046,807 of Sept. 6, 1977also shows the carbonylation of methyl acetate to produce aceticanhydride using noble metal compound catalysts and iodides and shows theuse of triphenylphosphine as a promoter alone or in combination withcobalt acetate. Belgian Pat. No. 839,321 shows a process for thepreparation of ethylidene diacetate by the reaction of carbon monoxideand hydrogen upon methyl acetate or dimethyl ether using a Group VIIInoble metal catalyst in the presence of an iodide or bromide and in thepresence of a promoter which may be a phosphine, an arsine or a stibine.In some cases, acetic anhydride is produced as a by-product.

More recently, U.S. Pat. No. 4,115,444 dated Sept. 19, 1978 discloses animproved process for preparing carboxylic acid anhydrides, includingacetic anhydride, wherein specified esters and/or ethers arecarbonylated in the presence of Group VIII noble metals or theircompounds in a system containing an iodide or a bromide and in thepresence of a promoter of at least one metal of Groups IVB, VB and VIBor a non-noble metal of Group VIII or their compounds, in combinationwith an organo-nitrogen compound or an organo-phosphorus compoundwherein the nitrogen and phosphorus are trivalent.

It is an object of the present invention to provide a further improvedprocess for the manufacture of acetic anhydride and, in particular, toprovide an improvement in the process of U.S. Pat. No. 4,115,444.

In accordance with the invention, methyl acetate and/or dimethyl etherare carbonylated under substantially anhydrous conditions in thepresence of a Group VIII noble metal catalyst, in the presence of ahalide which is an iodide or a bromide, and in the presence of promoterscomprising hafnium in metallic or zero valent form in combination withan organo-phosphorus compound or an organo-nitrogen compound wherein thenitrogen and phosphorus are trivalent, or an arsine.

It has been discovered that this catalyst-multiple promoter system makespossible surprisingly increased reaction rates. The rate of reaction andthe product concentration per unit of time realized from thiscatalyst-multiple promoter combination have been found to beunexpectedly exceptionally high.

The Group VIII noble metal catalyst, i.e., iridium, osmium, platinum,palladium, rhodium and ruthenium, can be employed in any convenientform, viz., in the zero valent state or in any higher valent form. Forexample, the catalyst to be added may be the metal itself in finelydivided form, or as a metal carbonate, oxide, hydroxide, bromide,iodide, chloride, lower alkoxide (methoxide), phenoxide or metalcarboxylate wherein the carboxylate ion is derived from an alkanoic acidof 1 to 20 carbon atoms. Similarly, complexes of the metals can beemployed, for example the metal carbonyls, such as iridium carbonyls andrhodium carbonyls, e.g., hexarhodium hexadecacarbonyl, or as othercomplexes such as the carbonyl halides, e.g., iridium tri-carbonylchloride [Ir(CO)₃ Cl]₂ or chlorodicarbonyl rhodium dimer, or theacetylacetonates, e.g., rhodium acetylacetonate Rh(C₅ H₇ O₂)₃. Includedamong the catalysts listed above are complexes of the Group VIII noblemetal with organic promoter ligands derived from the organic promotershereinabove described. It will be understood that the foregoingcompounds and complexes are merely illustrative of suitable forms of theGroup VIII noble metal catalyst and are not intended to be limiting.

As previously indicated, the hafnium is employed in metallic or zerovalent form, hafnium in this form having been surprisingly found toexhibit activity not shared by hafnium compounds. Moreover, the hafniummetal requires the presence of acetic acid in order for its surprisingand unexpected activity to be realized. Furthermore, it has been foundthat the hafnium is effective with an arsine or organo-phosphorus ornitrogen compounds as defined below but is inactive with stibines eventhough stibines are often grouped with arsines and organo-phosphoruscompounds such as phosphines.

The arsines employed in accordance with this invention have the formula:##STR3## wherein R and R¹ are monocyclic aryl groups or alkyl groups andR² is the radical ##STR4## a monocyclic aryl group or an alkyl group,and wherein R³ and R⁴ are each a monocyclic aryl group or an alkylgroup. It is preferred that at least one of R, R¹ and R² is a monocyclicaryl group, n being zero or a digit from 1-20. The alkyl groups in theforegoing formula are preferably lower alkyl groups of 1-8 carbon atoms,such as methyl, ethyl, propyl, butyl, hexyl and octyl, especially 1-4carbon atoms, but may contain as many as 20 carbon atoms. Similarly, thearyl group is preferably phenyl but may be phenyl substituted with oneor more alkyl groups containing one or more carbon atoms, e.g., up to 6carbon atoms, such as methyl, ethyl, propyl, butyl, and hexyl. Thephenyl groups can also be substituted with other non-reactivesubstituents such as halo, e.g., chloro, or cyano, and the like. Typicalexamples of the arsines used in the process of this invention includetriphenylarsine, tri-p-tolylarsine, dimethylphenylarsine,methyldiphenylarsine, methylditolylarsine, dimethylchlorophenylarsine,dimethylcyanophenylarsine, bis-(diphenylarsino) methane,bis-(diphenylarsino) ethane, bis-(diphenylarsino) propane,bis-(diphenylarsino) butane, tetraphenyl bi-arsine, triethyl arsine, andthe like. Most preferred are the alkyl diaryl arsines, especiallymethyldiphenylarsine.

The nitrogen and phosphorus promoters can, in a broad sense, be anyorgano-nitrogen or organo-phosphorus compound wherein the nitrogen andphosphorus are trivalent. Preferably, however, the organo-nitrogenco-promoter is an amine, especially a tertiary amine of the formula##STR5## wherein R⁵, R⁶ and R⁷ are the same or different and are alkyl,cycloalkyl, aryl or acyl groups which may be substituted bynoninterfering groups, preferably having up to 20 carbon atoms, such astrimethylamine, triethylamine, triphenylamine, ethylenediamine,tetraacetic acid, and the like, or a heterocyclic amine such aspyridine, picoline, quinoline, methylquinoline, hydroxy quinoline,pyrrole, pyrrolidine, pyrrolidone, and the like, or an imidazole, suchas imidazole, methyl imidazole and the like, or an imide of a carboxylicacid which may be monobasic or polybasic and which may be aliphatic oraromatic and preferably contain up to 20 carbon atoms, such as aceticacid, succinic acid, phthalic acid, pyromellitic acid, e.g.,succinimide, phthalimide and pyromellitic diimide, or a nitrile or anamide which may be aliphatic or aromatic and preferably contains up to20 carbon atoms, e.g., acetonitrile, N,N-dimethylacetamide, hexamethylphosphoric triamide, and like imides, nitriles, and amides, or an oximesuch as cyclohexanone oxime, and the like. It will be understood,however, that higher molecular weight promoters, e.g., polymeric formsof the organo-nitrogen compounds, may be used such as polyvinylpyridine,polyvinyl pyrrolidone, and the like.

The organo-phosphorus co-promoter is preferably a phosphine of theformula: ##STR6## wherein R⁸ and R⁹ may be the same or different and arealkyl, cycloalkyl, aryl groups, amide groups or halogen atoms and R¹⁰ isan alkyl, cycloaklyl, aryl, amide group, a halogen atom or the radical##STR7## R¹¹ and R¹² each being an aryl group or an alkyl group and nbeing zero or a digit from 1-20. Preferably, the groups contain up to 1to 20 carbon atoms in the case of alkyl and cylcloalkyl groups and 6 to18 carbon atoms in the case of aryl groups. Typical phosphines includetrimethylphosphine, tripropylphosphine, tricyohexylphosphine andtriphenylphosphine.

Although it is preferred that the organic promoters be added separatelyto the catalyst system, it is possible to add them as complexes with theGroup VIII noble metal such as the trichloro trispyridine rhodium,tris(-triphenyl phosphine) rhodium, chlorotris(triphenyl phosphine)rhodium, and chlorocarbonyl bis (triphenyl phosphine) rhodium previouslymentioned. Both free organic promoters and complexed promoters can alsobe used. Indeed, when a complex of the organic promoter and the GroupVIII noble metal is used, it is desirable to add free organic promoteras well.

In carrying out the process of the present invention, carbon monoxide isreacted with methyl acetate or dimethyl ether to produce aceticanhydride, the carbonylation taking place in the presence of an iodideor bromide, e.g., a hydrocarbyl halide, especially a lower alkyl halide,which is an iodide or bromide, such as methyl iodide. Thus, aceticanhydride can be effectively prepared in a representative case bysubjecting methyl acetate or dimethyl ether to carbonylation in thepresence of methyl iodide. In all cases, the carbonylation is carriedout under substantially anhydrous conditions in the presence of thecatalyst-multiple promoter system described above. As indicated, anester-ether mixture can be carbonylated if desired.

It will be understood that the hydrocarbyl halide may be formed in situand the halide may thus be supplied to the system not only as thehydrocarbyl halide but the halogen moiety may also be supplied asanother organic halide or as the hydrohalide or other inorganic halide,e.g., a salt, such as the alkali metal or other metal salts, or even aselemental iodine or bromide. Following the reaction the organiccomponents of the reaction mixture are readily separated from oneanother, as by fractional distillation.

The above-described reactions can be expressed as follows:

    CO+CH.sub.3 COOCH.sub.3 →(CH.sub.3 CO).sub.2 O      (1)

    2CO+CH.sub.3 OCH.sub.3 →(CH.sub.3 CO).sub.2 O       (2)

The more volatile alkyl halide and any unreacted ether or ester whichare present in the final reaction mixture can be readily removed, as bydistillation, for recycling, and the net yield of product issubstantially exclusively the desired carboxylic anhydride. In the caseof liquid-phase reaction, which is preferred, the organic compounds areeasily separated from the metal-containing components, as bydistillation. The reaction is suitably carried out in a reaction zone towhich the carbon monoxide, the ester or ether, the halide and the noblemetal catalyst and the promoters are fed. No water is produced in theabove-described reactions and anhydrous or substantially anhydrousconditions are employed.

In carrying out the process of the invention, a wide range oftemperatures, e.g., 25° to 350° C. are suitable but temperatures of 100°to 250° C. are preferably employed and the more preferred temperaturesgenerally lie in the range of 125° to 225° C. Temperatures lower thanthose mentioned can be used but they tend to lead to reduced reactionrates, and higher temperatures may also be employed but there is noparticular advantage in their use. The time of reaction depends largelyupon the temperature employed. The reaction is carried out undersuperatmospheric pressure but excessively high pressures, which requirespecial high-pressure equipment, are not necessary. In general, thereaction is effectively carried out by employing a carbon monoxidepartial pressure which is preferably 15 to 1,000 p.s.i., and mostpreferably 30 to 700 p.s.i., although carbon monoxide partial pressuresof 1 to 10,000 p.s.i. can also be employed. By maintaining the partialpressure of carbon monoxide at the values specified, adequate amounts ofthis reactant are always present. The total pressure is preferably thatrequired to maintain the liquid phase and in this case the reaction canbe advantageously carried out in an autoclave or similar apparatus.Preferably, the reaction product is introduced into a distillation zone,which may be a fractional distillation column, or a series of columns,effective to separate the hydrocarbyl halide and the ester or ether,free organic promoter and the product anhydride. The boiling points ofthese several compounds are sufficiently far apart that their separationby conventional distillation presents no particular problem. Likewise,the higher boiling organic components can be readily distilled away fromthe noble metal catalyst, the hafnium promoter, and any organic promoterwhich may be in the form of a relatively non-volatile complex. Thehydrocarbyl halide and the noble metal catalyst and the promoters, canthen be combined with fresh amounts of ester or ether and carbonmonoxide and reacted to produce additional quantities of anhydride.

The ratio of ester or ether to the halide in the reaction system canvary over a wide range. Typically, there are used 0.1 to 1,000 mols ofthe ester or ether per mol of halide, preferably 1 to 30 mols per mol.

The process is, as mentioned, carried out in the presence of acetic acidand, indeed, effective results require the presence of acetic acid inamounts of 1 to 75 wt. percent, preferably 10 to 30 wt. percent, of thetotal reaction mixture. Other organic solvents or diluents may be usedin addition to acetic acid, if desired, particularly when the reactanthas a relatively low boiling point, as in the case of dimethyl ether.The presence of a higher boiling solvent or diluent, which may be theproduct anhydride itself, e.g., acetic anhydride in the case of dimethylether, or which may be the corresponding ester, e.g., methyl acetate,again in the case of methyl ether, will make it possible to employ moremoderate total pressure. Alternatively, the additional solvent ordiluent may be any organic solvent which is inert in the environment ofthe process such as hydrocarbons, e.g., octane, benzene and toluene. Asolvent or diluent is suitably selected which has a boiling pointsufficiently different from the desired product in the reaction mixtureso that it can be readily separated, as will be apparent to personsskilled in the art.

The carbon monoxide is preferably employed in substantially pure form,as available commercially, but inert diluents such as carbon dioxide,nitrogen, methane, and noble gases can be present if desired. Thepresence of inert diluents does not effect the carbonylation reactionbut their presence makes it necessary to increase the total pressure inorder to maintain the desired CO partial pressure. The carbon monoxide,like other reactants should, however, be essentially dry, i.e., the COand the other reactants should be reasonably free from water. Thepresence of minor amounts of water such as may be found in thecommercial forms of the reactants is, however, entirely acceptable.Hydrogen which may be present in very small amounts as an impurity isnot objectionable and even may tend to stabilize the catalyst.

The metals employed may contain impurities normally associated with thecommercially available metal or metal compounds, and need not bepurified any further. Thus, the commercially available metal or metalcompound is suitably employed in the case of the Group VIII noble metalcatalyst and the commercially available hafnium metal may be employed asthe metal promoter.

Although it is preferred that the arsine, phosphorus or nitrogenpromoters be added separately to the catalyst system, it is possible toadd them as complexes with the Group VIII noble metal such aschlorotris-(triphenylarsine) rhodium, chlorocarbonyl (triphenylarsine)rhodium, hydridocarbonyltris-(triphenylarsine) rhodium, thecorresponding methyldiphenylarsine compounds,trichlorocarbonyl-bis-(triphenylarsine) rhodium, andtrichlorocarbonyl-bis-(methyldiphenylarsine) rhodium,trichlorocarbonyl-bis-(triethylarsine) rhodium, and the correspondingphosphorus and nitrogen compound complexes, and the like. Both freeorganic promoters and complexed promoters can also be used. Indeed, whena complex of the organic promoter and the Group VIII noble metal isused, free organic promoter may be added as well, if desired.

The amount of Group VIII noble metal catalyst is in no way critical andis not a parameter of the process of the invention and can vary over awide range. As is well known to persons skilled in the art, the amountof catalyst used is that which will provide the desired suitable andreasonable reaction rate. However, essentially any amount of catalystwill facilitate the basic reaction and can be considered acatalytically-effective quantity. Typically, however, the catalyst isemployed in the amount of 1 mol per 10 to 100,000 mols of ester orether, preferably 1 mol per 100 to 10,000 mols of ester or ether, andmost preferably 1 mol per 500 to 2,000 mols of ester or ether.

The quantity of hafnium promoter can vary widely. Typically, it is onemol per 10,000 mols of ester or ether, preferably it is used in theamount of 1 mol per 20 to 2,000 mols, most preferably 1 mol per 50 to500 mols of ester or ether. The quantity of organic promoter can alsovary widely but typically it is used in the amounts of 1 mol per 1 to10,000 mols of ester or ether, preferably 1 mol per 10 to 1,000, mostpreferably 15 to 200 mols of ester or ether.

In the working up of the reaction mixtures, e.g., by distillation, asdiscussed above, the hafnium promoter generally remains with the GroupVIII noble metal catalyst, i.e., as one of the least volatilecomponents, and is suitably recycled or otherwise handled along with thecatalyst. The organic promoter can also be recovered and recycled.

It will be apparent that the above-described reactions lend themselvesreadily to continuous operation in which the reactants and catalyst,preferably in combination with the promoter combination, arecontinuously supplied to the appropriate reaction zone and the reactionmixture continuously distilled to separate the volatile organicconstituents and to provide a net product consisting essentially of theacetic anhydride, with the other organic components being recycled and,in the case of liquid-phase reaction, a residual Group VIII noblemetal-containing (and promoter-containing) fraction also being recycled.In the case of such continuous operation, it will be apparent that thehalogen moiety remains in the system at all times subject only tooccasional handling losses or purges. The small amount of halogen makeupwhich may be needed from time to time is preferably effected bysupplying the halogen in the form of the hydrocarbyl halide but, aspointed out above, the halogen moiety may also be supplied as anotherorganic halide or as the hydrogen halide or other inorganic halide,e.g., a salt, such as the alkali metal or other metal salts, or aselemental iodine or bromine.

The following examples will serve to provide a fuller understanding ofthe invention, but it is to be understood that they are given forillustrative purposes only, and are not to be construed as limitative ofthe invention. In the examples, all percentages are by weight, unlessotherwise indicated.

In the examples, the various reactants and catalyst components arecharged to the reaction vessel which is then closed and brought to thereaction temperature indicated. The initial carbon monoxide partialpressure specified is the calculated value at reaction temperature atthe beginning of the reaction, i.e., at zero conversion. The totalpressure is maintained by introducing additional carbon monoxide as thereaction proceeds.

EXAMPLE I

A mixture of 80 wt.% methyl acetate and 20 wt.% acetic acid containing0.01 mol per liter of rhodium trichloride hydrate, 0.6 mol per liter ofmethyl iodide, 0.04 mol per liter of hafnium metal in powder form and0.16 mol per liter of triphenylarsine was heated at 160° C. in a stirredHastelloy pressure vessel, under an atmosphere of carbon monoxide(continuous total pressure 700 psig; initial partial pressure of carbonmonoxide 400 psig). G.C. (gas chromatography) analysis of the reactionmixture after a 6-hour reaction time showed it to contain 52.2% aceticanhydride, the balance being unreacted methyl acetate and the catalystand promoter components.

EXAMPLE II

Example I was repeated except that 0.16 mol per liter oftri-n-butylphosphine was used instead of the arsine. After 6 hours ofreaction, G.C. analysis showed the reaction mixture to contain 40.7%acetic anhydride.

EXAMPLE III

Example I was again repeated except that 0.16 mol per liter of pyridinewas used instead of the arsine. After 6 hours of reaction, G.C. analysisshowed the reaction mixture to contain 57.2% acetic anhydride.

COMPARATIVE EXAMPLE A

Example I was repeated except that the acetic acid was omitted from thereaction mixture. After 6 hours of reaction, G.G. analysis showed thereaction mixture to contain 4.3% acetic anhydride.

COMPARATIVE EXAMPLE B

Example I was again repeated but using 0.04 mol per liter of hafnium inthe form of hafnium iodide. G.C. analysis of the reaction mixture showedit to contain 18.9% acetic anhydride.

COMPARATIVE EXAMPLE C

In this case, Example I was again repeated except that 0.04 mol perliter of titanium in powder form was used instead of hafnium. G.C.analysis of the reaction mixture after a 6-hour reaction time showed itto contain 27.2% acetic anhydride.

COMPARATIVE EXAMPLE D

Again Example I was repeated but 0.04 mol per liter of niobium in powderform was used instead of hafnium. G.C. analysis of the reaction mixtureafter a 4-hour reaction time showed it to contain 9.1% acetic anhydride.

COMPARATIVE EXAMPLE E

Repeating Example I again but using 0.04 mol per liter of tantalum inpowder form instead of hafnium produced a reaction mixture after a6-hour reaction time which G.C. analysis showed to contain 15.4% aceticanhydride.

What is claimed is:
 1. A process for the preparation of acetic anhydride which comprises reacting carbon monoxide, a halide which is an iodide or bromide and a compound selected from the group consisting of a methyl acetate and dimethyl ether under substantially anhydrous conditions in the presence of 1 to 75 weight percent acetic acid, in the presence of a Group VIII noble metal catalyst and in the presence of a multiple promoter comprising metallic hafnium, and an organo-phosphorus compound or an organo-nitrogen compound wherein the phosphorus and nitrogen are trivalent, or an arsine of the formula: ##STR8## wherein R and R¹ are monocyclic aryl groups or alkyl groups and R² is the radical ##STR9## a monocyclic aryl group or an alkyl group, and wherein R³ and R⁴ are each a monocyclic aryl group or an alkyl group, n being zero or a digit from 1-20.
 2. A process as defined in claim 1, wherein the Group VIII noble metal is rhodium.
 3. A process as defined in claim 1, wherein the halide is a hydrocarbyl halide.
 4. A process as defined in claim 1, wherein the amount of acetic acid is 10 to 30 weight percent
 5. A process as defined in claim 1, wherein the amount of acetic acid is 10 to 30 weight percent, the Group VIII noble metal is rhodium and the halide is a hydrocarbyl iodide. 